The present invention relates to a positioning system employed in a processing machine and others.
A positioning device employed in the processing machine is desired to perform a speedy and precise positioning. Particularly, in a super-precision process and a semiconductor manufacturing device, it is necessary to perform a positioning in the unit of submicrons.
There has been a positioning system employed with a lead screw as a positioning mechanism. The lead screw is driven by a motor such as a pulse motor or a DC motor. The precision of the positioning by the pulse motor is decided by a minimum quantity of the feeding (a unit feed length) of the pulse motor.
In the positioning system with the lead screw driven by the pulse motor, the highest feed speed is decided by multiplying the minimum quantity of the feed by the highest response frequency of the pulse motor. If the minimum quantity is 0.1 .mu.m, the highest feed speed is 600 mm/min at most.
In a positioning system having the lead screw driven by a servomotor, a speed control of a motor driving circuit is limited in a small range and friction at various parts such as a table opposing the feeding motion. Accordingly, it is difficult to realize both a precise positioning and a high speed driving.
On the other hand, a reduction gear device is disposed between the motor and lead screw to obtain a minimum quantity of feed. However, the reduction gear device has backlashes. In case of a closed-loop control system, the backlashes cause an oscillation of the system, and in case of an opened-loop control system, the backlashes have influences on the accuracy of feeding, when the table returns. However, in a reduction gear device having small backlashes, it is hard to obtain a sufficient reduction gear ratio.
In a positioning system employed with a hydraulic cylinder, a maximum speed is decided by a rated flow rate of a servo valve and area of a piston, and a minimum quantity of feed is decided by a controllable minimum flow rate. Accordingly, it is difficult to provide a speedy and precise positioning.